The hard disk drive (HDD) for a disk of 3.5 to 1.8 inches or even 1.0 inch or less has been used in various fields of automobile products, home electrical appliances and audio appliances, etc. Therefore, the reduction of cost of hard disk drive and the mass production thereof have been requested and the large memory capacity thereof has been also requested.
In order to satisfy these requests, there is a tendency that the high density recording magnetic disk media of the vertical magnetic memory system, which has lately been put to practical use, has been employed in the above mentioned fields and spread rapidly.
The magnetic disk medium of the vertical magnetic memory system is used in a composite magnetic head having a TMR (tunnel magneto-resistance) head or a GMR (giant magneto-resistance) head, which is a memory medium separable from the head by 10 nanometer or less controllably.
Such magnetic disk medium generally includes a glass substrate, a soft magnetic layer formed on the glass substrate and a magnetic layer provided on the soft magnetic layer. Discrete tracks are formed in a discrete substrate by etching the magnetic layer. (Incidentally, the term “disk substrate” is used as a material of a magnetic disk to be mounted on a hard disk drive.)
The etching for forming grooves between tracks is performed through an uneven photo-resist film. The unevenness of the photo-resist film is formed by forming the photo-resist film on the magnetic layer of the disk substrate by using the nano-print lithography and pushing the photo-resist film with an uneven stamper. The track width of the discrete track formed by the dry etching through the uneven photo-resist film is 100 nm or less and the groove separating adjacent tracks is filled with a non-magnetic material in a later step.
Such technique is described in JP-2007-012119A and JP-2007-149155A, etc., and is well known.
The magnetic disk of this kind is called as a magnetic recording medium of the discrete track system (DTM) and is currently paid attention to a technique capable of realizing ultra high density recording exceeding 1 terabit/(inch)2 for 2.5 inches several years later. Further, the bit patterned medium (BPM) having discrete tracks, which are finely separated magnetically in the track direction, has been entered into the practical implementation step recently.
Since a magnetic film of the prior art magnetic disk used in HDD is formed on the whole surface of the medium, the prior magnetic disk is easily possible to record test data (test burst signal) in arbitrary track by a write head. Therefore, the read voltage characteristics, that is, the read characteristics profile (waveform), with respect to the moving distance of the read head crossing the track can be obtained easily by reading test data recorded in the track while moving the read head continuously in radial direction of the disk. With the profile of the read characteristics, the write sensitive width of the write head and the read sensitive width of the read head can easily be measured as the characteristic parameter of the composite magnetic head in the magnetic head test and, therefore, the composite magnetic head can be evaluated or tested.
FIG. 6 explains a prior art measuring method for measuring a write sensitive width of a write head and a read sensitive width of a read head as characteristic parameters of a magnetic head.
In FIG. 6, it is assumed that a write of test data in a designated track with write sensitive width Wa by a composite magnetic head (write head) has been completed already. In a read step of the test data, the test data is read by moving the composite magnetic head (read head) rightward in the drawing along a radial direction of the disk across the designated track.
In a position (1) shown in FIG. 6, a right side end of a read sensitive width Wb of an MR head (read head) corresponds to a left side end of the write sensitive width Wa of the test data. At this time, a gap (center line Cb) of the MR head can read the test data (the left side end thereof) written by the write head. In this case, the read voltage is still 0 (zero).
In order to simplify the description, the unit of the read voltage of the MR head is not [mV] but a ratio in a range between numerical value “0” and numerical value “1” under a maximum read voltage of the test data being 1. Incidentally, each of the sensitive widths Wa and Wb of the heads is determined by the gap width of the heads. The write sensitive width Wa of the write head (thin film inductive head) was usually in the order of several μm. In the DTM, the write sensitive width of the write head is in the order of 50 nm to 80 nm and the track width is 50 nm or less. Further, the track formed is eccentric. Therefore, even if the write sensitive width of the write head is close to the track width substantially, there is a problem that the track width becomes narrower than the write sensitive width of the write head in the data recording state.
At a position (2), the read sensitive width Wb of the MR head enters into the side of the write sensitive width Wa by Wb/2. Therefore, Wb/2 of the right side of the read sensitive width Wb becomes on the write sensitive width Wa. In this state, the read voltage becomes 0.5 when the test data is written uniformly. When the MR head is moved rightward further to a position (3), the read sensitive width Wb overlaps the write sensitive width Wa completely. Therefore, the maximum read voltage becomes 1.0. When Wa>Wb, the voltage in the width range (Wa−Wb) becomes 1.0 evenly and the read voltage becomes flat. Therefore, when the MR head is at a position (4), the read voltage is 1.0. As a result, it is possible to obtain the profile (waveform) of the read voltage characteristics having a center flat portion as shown by a thick solid line. Incidentally, the head parameter measuring method of this kind is described in JP-2000-231707A and known publicly.
When the track width becomes narrower than the write sensitive width Wa of the write head as in the DTM, the read head can not cross the whole write region determined by the write sensitive width even if the read head is moved in radial direction. Therefore, there is the problem that the profile of the read voltage characteristics shown in FIG. 6 can not be obtained. Further, since the read sensitivity width of the read head in the DTM becomes close the track width, it is impossible to obtain the profile having the center flat portion as shown in FIG. 6. Therefore, it becomes difficult to measure the write sensitive width of the write head and the read sensitive width of the read head.